(1) Field of the Invention
The present invention relates to a system and method for navigating a vehicle along a set route of travel in which an optimum route of travel from a starting point of the vehicle to a destination can be set.
(2) Description of the Background Art
Recently, various types of, so-called, navigation systems have been proposed in which a present position of the vehicle is sequentially derived on the basis of a direction information generated from a vehicular direction sensor and a distance information generated from a running distance sensor and the derived vehicular position is continuously displayed on a displayed road map.
A Japanese Patent Application First Publication No. Heisei 1-130299 published on May 23, 1989 exemplifies a previously proposed vehicular navigation system.
In the disclosed navigation system, such inherent characteristic data as a running distance, a time it takes for the vehicle to reach a destination, and a charge required for the vehicle to reach the destination is stored in a memory together with a regional road map data. It is herein noted that a position of an intersection or a branched road point in a road network is hereinafter referred to as a node.
The inherent characteristic data described above will specifically be explained below.
For example, in a first road region, the inherent characteristic data of a route from a first node to a fifth node via a third node are displayed aside a displayed regional road map having a plurality of nodes on a television screen. In a second road region, the inherent characteristic data of another route from the first node to the fifth node via a fourth node are displayed on the same screen. In a third road region, the inherent characteristic data of another route from the first node to the fifth node via the second and fourth nodes.
In the disclosed navigation system, when an arbitrary starting point and arbitrary destination are plotted, respectively, the inherent characteristic data are referred to so that a single route is calculated providing a shortest distance from the point of start to the destination, a single route is calculated providing a shortest required time, and a single route is calculated providing a least cost of travel, respectively.
A method for searching one of the routes on which the vehicle is to run from a road map data includes, e.g., a, so-called, dikisutora method in which a shortest distance between a plurality of road points is sequentially derived to search the vehicle running route.
In the previously proposed searching method utilizing the dikisutora method, the road network data of a district in which the vehicular starting point and the destination are placed, i.e., the position data on the intersections and the data on road segments connected to the respective intersections are referred to and, in addition to these data, the inherent characteristic data are referred to as well to determine an optimum route of travel from the start point to the destination. At this time, such a series of processings as described above are executed without consideration of pass time upon vehicle's start from the starting point and change in the running condition of the vehicle.
In the previously proposed searching method, even in a case where one of the road segments on which a passage of vehicle is limited or prohibited is present on the set route of travel, the series of processings related to the search for the optimum rouge are executed without a predicted time at which the vehicle is to pass through the road segment(s) on which the passage of vehicle is limited taken into consideration.
Therefore, in a case where the search for the optimum route including the search for the road segment(s) on which a traffic limitation is carried out for a specified time duration is executed, an appropriate determination of the route of travel and an optimum search for the route of travel on which the vehicle reaches the destination becomes difficult.
In addition, since, in the previously proposed searching method, a time difference between a time at which the vehicle has actually passed through an arbitrary point of location and a predicted time at which the vehicle is to pass through the arbitrary point of location in a running scheme is not considered and such a series of search processings as described above are executed, an appropriate determination on the road segment(s) on which the traffic limitation is carried out cannot be made and the search for the optimum route according to the change in the running condition of the vehicle becomes difficult.
Furthermore, since, in the previously proposed searching method, a recalculation of such a inherent characteristic data as the running distance and so on as recited above needs to be repeated so that the processing time becomes long. Consequently, the searching for the optimum route of travel becomes difficult in a very short period of time.